OPTICAL-OPTICAL DOUBLE RESONANCE SPECTROSCOPY OF SrOH: THE 2 Π(000) – 2 Π(000) AND THE 2 Σ + (000) – 2 Π 1/2 (000) TRANSITIONS J.-G. WANG, P. M. SHERIDAN, M. J. DICK, S. YU, P. F. BERNATH Department of Chemistry University of Waterloo

 Previous high-resolution studies have largely focused on the low-lying states: — X 2  + (SrOD, 88 SrOH) Anderson et al. (1992) — A 2  – X 2  + Brazier et al. (1985) — B 2  + – X 2  + (SrOD) Nakagawa et al. (1983)  Recently, moderate resolution spectra of the higher energy states have been measured : — C 2  – X 2  +, D 2  + – X 2  +, E 2  + – X 2  +, F 2  – X 2  + Beardah et al. (1999) — B′ 2  + – X 2  + Beardah et al. (2003) SrOH Previous Work ~ ~~ ~ ~ ~ ~~~~~ ~ ~ ~~

 The C 2  state of SrOH is of interest because the dipole moment of CaF in this state was found to be large (~9D) compared to the lower energy states. This was rationalized as a polarization of the unpaired electron towards the ligand in this state.  The B' 2  + state of SrOH is of interest because no obvious analog of this state has been found in strontium fluoride and no theoretical calculations have characterized this state.  High-resolution laser excitation spectroscopy of high energy states in the UV region is difficult using one laser excitation fluorescence spectroscopy. OODR spectroscopy is an alternative technique that can more effectively access these higher energy states. Motivation ~ ~

 Pump laser:  Broad-band linear dye laser (DCM)  A 2  – X 2  + transition  Fixed wavelength  Probe laser:  Single mode Ti:Sapphire laser  C 2  – A 2  transition  Scan wavelengths  Measure UV fluorescence from the C 2  to X 2  + state vs. probe laser wavelength OODR Excitation Scheme: C 2  – A 2  Transition Broad band laser DCM Single mode laser Ti:Sapphire OODR fluorescence ~~ ~~ ~~ ~~

Experimental Details  SrOH was produced by the reaction of Sr (Broida oven source) and H 2 O 2  Pump and probe lasers were overlapped above Broida oven in area of maximum molecular production  UV fluorescence only collected by placing a 500 nm blue pass filter in front of the PMT

Optical-Optical Double Resonance Experimental Setup

Q 12 (ef) and P 11 (ee) branch heads Pump laser: (14541 cm -1 ) OODR Spectrum : Transition Q 11fe P 11ee, P 11ff R 11ee, R 11ff  Broad band pump of band heads: populate many A 2  1/2 rotational levels ~

Expanded OODR Spectrum: Transition C 2  1/2 A 2  1/2 ~ ~

 Combination differences with the A 2  state helped make assignments  454 lines measured in 10 branches of the C 2  – A 2  transition  Combined fit: pure rotational (X 2  + ), A 2 Π – X 2 Σ + transition, and current C 2 Π – A 2 Π transition data Results and Analysis ~ ~~ ~~~ ~~

X2+X2+ A2A2 B2B2 C2C2 r 0 (SrO)Å r 0 (OH)Å  In CaF, the C 2  state has the largest metal-ligand bond length among the 4 lowest energy states. This elongation is thought to be due to the polarization of the unpaired electron towards the anionic ligand.  Constants for A 2  and C 2  states of SrOD have not been measured; therefore the r 0 (OH) bond length is fixed to the X 2  + state value.  By fixing the C 2  r 0 (OH) bond length the X 2  + state, the C 2  state was found to have the shortest Sr-O bond length, opposite the case of CaF. Bond Lengths ~~ ~ ~~~~ ~~~

StateA spin orbit (cm -1 ) A2A2 (61) C2C2 (11)  Spin-orbit constant is 10 times larger in the A 2  state than in the C 2  state  Implies that the unpaired electron in the A 2  and C 2  states posses different atomic orbital character  A 2  state dominant contribution by 5p  and 4d  Sr + atomic orbitals  Higher energy Sr + atomic orbitals (6p  and 5d  ) have smaller atomic spin orbit constants; a greater contribution from these oribitals would decrease the spin-orbit constant in the C 2  state Spin Orbit Constant ~ ~ ~~ ~ ~ ~ ~

 Assuming pure precession model: Using E D  = cm -1 and l effective = 1  A and B states – 5p π /5p σ unique perturber pair  C and D states – 6p π /6p σ unique perturber pair (cm -1 )MeasuredCalculated p (75) q (84)x x10 -4  - Doubling Constants ~~ ~~

Q 12 (ef) and P 11 (ee) branch heads Pump laser: (14541 cm -1 ) OODR Spectrum : Transition P 11 (ee) Q 11 (ef), P 21 (ff) R 11 (ee), Q 21 (fe) R 21 (ff),

B′ 2  + - A 2  1/2 Pump laser ~ cm -1 OODR Spectrum: B′ 2   – A 2  1/2 Transition ~ ~  Pump laser was around Q 11(fe) (J=10.5 and 50.5) A 2   – X 2  + ~ ~~~

 Rotational assignments were made using lower state combination differences with the A 2  state  231 lines were measured in 6 branches  A simultaneous least-squares fit was performed with the pure rotational (X 2  + ), A 2  – X 2  + transition, and current B' 2  + – A 2   transition data Results and Analysis ~  Small  implies B′ 2  + state arises from an 6s atomic orbital (X 2  + ~5s)  Large value of B may suggest shortening of metal-ligand bond ~ ~ ~ ~~ ~

 OODR spectra of the D 2  + ─ A 2  transition have now been measured and are under analysis  Recently we have recorded high-resolution spectra of the A 2  – X 2  + and C 2  – A 2  transitions of SrOD  r 0 (OH) for the C 2  state is too small; same value observed for X 2  + state of MgOH  explained in terms of large amplitude bending motion  Beardah et al. also observed large activity in C 2  state bending mode Work In Progress ~ ~~ ~ ~ ~ ~ ~~ ~~ Bond lengths (Å) SrOH A2A2 C2C2 r 0 (SrO) r 0 (OH)

Funding: NSERC Acknowledgements